In VHS recording, the luminance and chrominance information contained in a composite video signal are separated from each other. The chrominance information supplied for recording is mixed with one of four phases of a 4.21 MHz carrier in a down conversion that generates a color-under signal that comprises quadrature-amplitude-modulation (QAM) sidebands of a suppressed color-under carrier with a nominal frequency of 629 kHz. The phasing of the 4.21 MHz carrier wave is selected on a line-by-line basis, the selection signal being generated by decoding counts of the horizontal and vertical synchronizing pulses separated from the composite video signal. The luminance information is used to frequency-modulate a higher frequency luminance carrier wave. In the resulting FM signal the sync tips are at about 3.4 MHz; black level is at about 3.7 MHz; and white level is at about 4.4 MHz. This FM signal is added to the color-under sidebands as a bias frequency, and the resulting sum signal is pre-emphasized and used to record the video tape vertically scanned by the VCR type transport.
The playback electronics of known VCRs include filtering that separates the signal reproduced from the electromagnetic tape into two components. The component separated by a high-pass or band-pass filter is a luma carrier frequency-modulated in accordance with luminance and synchronizing signals; and the gain of the luminance and synchronizing signals recovered by demodulating the frequency-modulated luma carrier depends on the frequency-deviation of the carrier, rather than on its amplitude. The component separated by a low-pass filter is the color-under signal. Color saturation depends on the amplitude of the color-under signal, because the color-difference signals are derived by demodulating suppressed-carrier QAM sidebands recovered by upconverting the color-under signal. So, in order properly to track color-difference signals derived from the color-under signal with recovered luminance signal, the color-under signal is passed through a variable gain control amplifier, the gain of which is controlled by an automatic gain control (AGC) loop responding to the amplitude of color burst. This type of AGC loop is sometimes more specifically referred to as an automatic color control (ACC) loop. In known VCRs the amplitude of color burst is detected after the color-under signal is up-converted to the normal chrominance band, centered at 3.58 MHz in a VCR for recording NTSC television signals. The reproduced chrominance signal is combined with the reproduced luminance signal to form a composite signal that can be supplied to a television receiver, either directly or as modulate onto a radio-frequency picture carrier.
In newer types of VCRs, such as one of the type generally described in U. S. Pat. No. 5,113,262 entitled VIDEO SIGNAL RECORDING SYSTEM ENABLING LIMITED BANDWIDTH RECORDING AND PLAYBACK issued May 12, 1992 to C. H. Strolle et alii, time-base correctors are used for the luminance signal, as demodulated from the frequency-modulated luma carrier, and for the color-under signal. These time-base correctors employ digital memory, written in accordance with a time-base derived from the signals recovered during playback of the tape, and read in accordance with a more stable time-base so that transversal digital filtering can be done over adjacent scan lines. The luminance signal and the color-under signal must be digitized before they can be written into the digital memory; and in order effectively to utilize the limited number of bits of resolution (e.g., eight) in a cost-effective analog-to-digital converter, it is a practical necessity to gain-control the color-under signal prior to its digitization. The variable delay time base correction and the large number of processing steps between the analog-to-digital converter and the up-conversion of the color-under signal to the normal chrominance band for composite video signal tend to cause tracking problems in an ACC loop detecting burst amplitude after that up-conversion, the inventor's co-workers found.
The inventor's co-workers considered locating a digital-to-analog converter and a up-converter just for implementing ACC after the analog-to-digital converter, but before the time-base corrector. However, the up-converter requires filtering to reject image frequencies. Practically speaking, such filtering requires the use of inductive elements, which are better avoided, particularly when monolithic integrated circuitry is to be used in the playback electronics inasfar as possible.
The inventor's co-workers considered performing the up-conversion and filtering in the digital regime, followed by digital-to-analog conversion of the ACC signal. This was found to be undesirable since it required clocking frequencies higher than those needed for digitizing and digitally filtering the limited-bandwidth luminance and color-under signals recovered during playback from recorded video tape.
Accordingly, an ACC loop detecting burst amplitude directly from the color-under signal without up-conversion was sought by the inventor for use in VCR playback electronics. Implementing such an ACC loop is not straight-forward design, however. In the prior art ACC scheme, up-conversion produces a chroma reference signal, or color burst, having a prescribed number of cycles (i.e., eight or nine) during a chroma burst interval which can readily be synchronously detected to provide accurate detection of the level to which the amplitudes of the color-difference signals should be referred. In-order to eliminate the up-conversion used in the prior art, the inventor perceived, a detector circuit was needed which could produce an accurate representation of the amplitude of the amplifier output by sensing and/or rectifying the cycle and a part in each of the down-converted color bursts and still be capable of producing an output suitable for controlling the gain of the ACC'd amplifier. The inventor could find no such detector in the prior art. In order to realize the system invention herein described and claimed, the inventor developed a new signal detection circuitry which can accurately detect the amplitude of very few cycles of low-amplitude signals. This signal detection circuitry is described herein and in an application entitled CLIPPER CIRCUITRY filed before the U.S. Patent and Trademark Office on even date herewith, which application claims the new signal detection circuitry.